Added warning options for Clang
Added warning options for Clang

Clang knows way more warnings than GCC, and they are much easier to
activate. Currently the code generates many warnings, therefore they are
individually disabled.

lavabit/libdime,greyg00s/libdime,lavabit/libdime,lavabit/libdime,greyg00s/libdime,lavabit/libdime